Permafrost in the Cretaceous supergreenhouse

Earth’s climate during the last 4.6 billion years has changed repeatedly between cold (icehouse) and warm (greenhouse) conditions. The hottest conditions (supergreenhouse) are widely assumed to have lacked an active cryosphere. Here we show that during the archetypal supergreenhouse Cretaceous Earth, an active cryosphere with permafrost existed in Chinese plateau deserts (astrochonological age ca. 132.49–132.17 Ma), and that a modern analogue for these plateau cryospheric conditions is the aeolian–permafrost system we report from the Qiongkuai Lebashi Lake area, Xinjiang Uygur Autonomous Region, China. Significantly, Cretaceous plateau permafrost was coeval with largely marine cryospheric indicators in the Arctic and Australia, indicating a strong coupling of the ocean–atmosphere system. The Cretaceous permafrost contained a rich microbiome at subtropical palaeolatitude and 3–4 km palaeoaltitude, analogous to recent permafrost in the western Himalayas. A mindset of persistent ice-free greenhouse conditions during the Cretaceous has stifled consideration of permafrost thaw as a contributor of C and nutrients to the palaeo-oceans and palaeo-atmosphere.

Permafrost is an amplifier of climate change 1,2 , emitting CO 2 and CH 4 from bacterial carbon degradation as permafrost thaws 3 , and providing nutrients and carbon to aquatic ecosystems 4,5 . But evidence of permafrost in the pre-Quaternary geological record ('deep time') is limited, as is geological evidence of continental ice during supergreenhouse periods. Intriguingly, however, a growing body of evidence suggests that cryospheric conditions developed at different times during the Cretaceous supergreenhouse.
Most evidence of a Cretaceous cryosphere comes in the form of marine ice-rafted debris (IRD) from the southern 6,7 and northern hemispheres [8][9][10][11] , as well as from the recognition of marine glendonites (calcite pseudomorphs after ikaite) indicating cold temperatures during formation [12][13][14][15] . Additional evidence includes Cretaceous landforms in Yukon and Alaska 16 , patterned ground in China 17 , and ice-rafted dropstones in desert oases indicating colddesert conditions in interdunes similar to those in the Badain Jaran Desert 11 . Recently, the discovery of ultra-depleted hydrogen isotopes from Antarctica has suggested glaciation of the South Pole during the Late Cretaceous 18 , and there is isotopic evidence for continental ice sheets in China during the Early Cretaceous 19 . Collectively, these studies hint at the likely occurrence of Cretaceous periglacial and permafrost environments in polar and high-altitude regions.
Here we report the former occurrence of Cretaceous permafrost in a plateau desert in China analogous to modern permafrost in the Western Himalayas (Fig. 1a). This raises new questions on the relative role of allogenic controls on Mesozoic palaeoclimates, including a cryosphere and the widespread release of nutrients to the palaeo-Arctic realm. The identification of past permafrost is crucial for understanding climate dynamics and the ocean-continent coupling during supergreenhouse conditions. Of particular importance is the response of high-altitude permafrost thaw due to global warming.
Permafrost thaw in the eastern Tibetan Plateau between 1969 and 2017, has increased by a factor of 40, with 70% of the thawed area forming since 2004, amplifying large-scale permafrost climate feedbacks 20 .

Cretaceous permafrost wedges and host desert dunes
Early Cretaceous westerly winds blew in eastern China 21 forming gigantic aeolian dunes (>352 m high) 22  Sandstone wedges have been identified in three different outcrops of the Luohe Fm. Outcrop one (Fig. 1b, 5) shows 10 wedges concentrated in two discrete horizons bounding three draa successions (Fig. 1c, d).
The sandstone wedges from the Cretaceous Luohe Fm show evidence of both primary and secondary infilling during the formation of thermal contraction crack wedges in permafrost and periglacial environments (Tables 2 and 3  Primary infilling of open thermal contraction cracks by aeolian sand is indicated by vertical to subvertical lamination within many of the sandstone wedges. Individual sandstone veins that branch away from the toes of the wedges represent individual crack infills. The same sedimentary structures typify cryogenic veins and wedges of primary infilling in present-day Arctic and Antarctic regions 28 , mid-latitude Pleistocene permafrost regions 29,30 , and in Proterozoic sandstones in Australia 31,32 . Secondary infilling with sand of voids created by the melt of original ice veins or small ice wedges within the overall wedge forms is indicated by several lines of evidence. First, small normal faults, some step-like, in strata adjacent to some wedges, indicate extensional faulting during the melt of ice 33 . Second, collapse structures 34 , and involutions in the upper part of some wedge infills indicate subsidence of adjacent sediment from the sides or roof during the melt of ice veins 35,36 . Third, fallen and rotated intraclasts were derived from the host sediments once adjacent to the upper margins of the original wedge 33 . Fourth, some parts of the infills appear massive, either because secondary infilling has disrupted the original primary lamination or because the lamination did not develop due to very uniform particle size (Fig. 2, and Supplementary Fig. 6).
Some sandstone wedges show a polygenic infill characterized by multiple wedges crossing horizontal lamination preserved in the wedges (Fig. 2). These complex patterns and superimposed wedges are similar to those in Upper Pleistocene wedges in western Europe (ref. 33, and references therein) and northwest Russia 37 . Superimposition results from the reactivation of thermal contraction cracking when the appropriate thermal conditions resume after a period of inactivity 38 .
Collectively, the sedimentary properties of the sandstone wedges in the Luohe Fm indicate that many of the wedges represent composite-wedge pseudomorphs, because they have composite infillings that comprise evidence for both primary and secondary infilling 26,39 (Figs. 1b, 2, and Supplementary Fig. 6). Wedges lacking evidence for secondary infilling are interpreted as relict sand wedges. Although small sand veins and wedges up to 0.5 m wide and up to 1.2 m deep may develop in regions of permafrost and deep seasonal frost 40 , the presence of composite-wedge pseudomorphs strongly supports the interpretation that they developed in a permafrost environment. This interpretation is also consistent with the dimensions of the sandstone wedges (up to at least 2 m high, and up to~1 m wide), dimensions that are common in modern permafrost environments 28 but have not been demonstrated to form purely under conditions of seasonal freezing. Overall, therefore, it is highly likely that the sand-

Recent permafrost analogue from the Western Himalayas
The evidence of past permafrost in a desert basin during the Cretaceous supergreenhouse period poses an apparent conundrum, but one that can be explained by reference to a modern analogue of a high-altitude aeolian-permafrost system at Qiongkuai Lebashi Lake, Xinjiang Uygur Autonomous Region, in the western Himalayas, China (Figs. 1a, 3-6, Supplementary Note 5 and Supplementary Fig. 10). Here, evidence from satellite imagery of permafrost persisting in the plateau lake-aeolian system for a period of 8 years ( Fig. 3c-f, and Supplementary Fig. 10) corroborates the permafrost modelling for the study area 42,43 (Fig. 5), and the measured altitudes for the aeolian dunefield and associated frozen oases of 3308 m above sea level (asl) (Figs. 4, and 6a-d) agrees with the probability of permafrost occurrence at this altitude 44 (Fig. 6a-d). Satellite imagery and topographic elevation sections of the study area (Fig. 6a) show that the terrain near Qiongkuai Lebashi Lake varies in altitude from 3.3 to 5.1 km (asl) (Fig. 6a).
The development of Cretaceous permafrost composite wedges and sand wedges associated with major bounding surfaces study sites in the Ordos Basin (Cretaceous permafrost) and the Qiongkuai Lebashi Lake (recent permafrost analogue). The Digital Elevation Model (DEM) data were downloaded from the Shuttle Radar Topography Mission (SRTM) on the USGS EarthExplorer (https://earthexplorer. usgs.gov/). Image was generated by using the Global Mapper 18.0 (Blue Marble Geographics) programme based on the Digital Elevation Model data. b Permafrost sandstone wedges horizon covered and preserved by downlapping aeolian dune toeset sandstones. c Field photograph of two wedge horizons hosted in aeolian dune sandstones of the Luohe Fm. d Ten wedges are identified (labelled wedge 1 "w1" to wedge 10 "w10" in c). Detailed sedimentological observations of the permafrost wedges can be seen in Fig. 2 and Supplementary Fig. 6. Aeolian architecture in d is based on the recognition of aeolian bounding surfaces hierarchy 25 ; "SS" aeolian supersurface; "IS" interdune surface; "S" superimposition surface; "R" reactivation surface. Wedges are marked in blue colour. See the enlarged image in Supplementary Fig. 5.    orthogonal (rectangular) thermal contraction crack polygons adjacent to the western and southern shorelines (Fig. 3). The lake ice looks to be melting, with ice-free (very dark) areas at the lake margin ( Fig. 3a and  b). The high transparency of the lake ice with no snow cover allowed good visibility of lake bottom permafrost polygons (Fig. 3).
In addition to the large-scale polygonal patterns, small-scale patterns are visible in the form of persistent rounded structures ( Fig. 3e and f). These structures have dimensions and shapes similar to those of earth hummocks, which are commonly observed in arctic permafrost terrain 45,46 . The same hummocks are visible in 2004 and 2012 associated with the permafrost polygons ( Fig. 3c-f), providing further evidence of permafrost persisting beneath the bottom of the shallow plateau lake. Such persistence of hummocks and polygons indicates beyond reasonable doubt that these parts of the lake were sufficiently shallow to freeze to the bottom in winter, enabling permafrost to persist beneath a lake-bottom active layer (Fig. 3). As a result, the patterned ground was preserved.
Lastly, frozen oases (interdunes) also show polygonal patterns ( Fig. 4a-g). Considering that both the shallow lake water and the interdune waters are frozen, and permafrost patterned ground is preserved beneath parts of the lake, then the water table in the aeolian dunefield must be also frozen, defining permafrost in the subsurface of the aeolian dunefield.
The occurrence of discrete horizons showing permafrost sandstone wedges (Figs. 1b-d, 2, and Supplementary Fig. 5) in a Cretaceous plateau aeolian dunefield is consistent with a long-lasting permafrost plateau aeolian system similar to that observed recently in the Qiongkuai Lebashi Lake area, Xinjiang Uygur Autonomous Region, China, where during transgressive periods the plateau lake favoured permafrost persisting in aeolian interdunes ( Fig. 4d-g). Well-defined permafrost horizons recorded in the lower Cretaceous Luohe Fm ( Fig. 1c and d, and Supplementary Fig. 5) attest to a similar process, where persistent plateau freezing conditions allowed the development of composite wedges and sand wedges in an aeolian dunefield. The occurrence of these permafrost wedge horizons associated with aeolian supersurfaces is further evidence of the temporal relationship among permafrost development and a variable phreatic level controlled by allogenic forcing 47 , as demonstrated in the satellite images of the recent analogue from the western Himalayas ( Fig. 4b-g).
The satellite imagery evidence of modern permafrost at Qiongkuai Lebashi Lake is consistent with a map that combines a digital elevation model (DEM) with modelled permafrost probability (PER-PROB) data from refs. 42,43. The map indicates that this area is probably underlain by permafrost; PERPROB values of 0.5-0.9 are classified as discontinuous permafrost, and values of >0.9 as continuous permafrost 43 (Figs. 5a-d, and 6c).
Regionally, the statistical distribution of modern high-altitude permafrost, glaciers, and the zero-degree quantiles for high-mountain Asia (Fig. 6b) 44 indicates that permafrost presently underlies 2-3.7% of the terrain at altitudes matching those of the cold aeolian dunefields and surrounding relief (3.3-3.6 km asl) and 7.3-11.8% of the surrounding glaciated mountain ranges (4.5-5.2 km asl) (Fig. 6a, b). The occurrence of Cretaceous permafrost in the plateau aeolian system supports the palaeotemperature modelling for this area for this time (ca. 0°C) 48 ( Supplementary Fig. 2b), and two scenarios are proposed for the Cretaceous palaeolatitude of the plateau desert system in the Ordos Basin: 53°N for the Barresian-Valanginian 48 , and 32.6-41.0°N for the Early Cretaceous 49,50 (Fig. 6e). A palaeolatitude of 53°N for the Cretaceous permafrost is similar to the latitudinal distribution of modern permafrost and glaciers in the North Asia area defined by the IPCC, and a palaeolatitude of 41°N for it is similar to the latitudinal distribution of permafrost and glaciers in the High Mountain Asia area defined by the IPCC (Fig. 6e). The 53°N scenario suggests an elevation mode of past permafrost at 899 m asl (range 186-3841 m asl), with permafrost underlying 17% of the surface (Fig. 6e). The 41°N scenario yields an elevation range for permafrost of 2196-6204 m asl, with a mode of 4672 m asl, and permafrost underlying 12% of the surface (Fig. 6e).

Permafrost astrochronology and the orbital control on the Hauterivian cold snap
The revised magnetostratigraphic studies were based on the geomagnetic polarity time scale (MHTC12) 51  Some notable low-power eccentricity (E7 and E8) occurs in the 405-kyr Gaussian bandpass filter (Fig. 7). The eccentricity signal during this period also became weaker in the evolutionary fast Fourier transform (FFT) spectral analysis of Well Wuqi (Fig. 7). In addition, an extended period of low-amplitude variability in obliquity is identified in the lower part of the Luohe Fm (ca. 132.49-132. 17 Ma) (Fig. 7).
A similar phenomenon has been documented for the Mi-1 glaciation event 52 . The lower the obliquity, the less solar radiation the polar regions receive, which favours the creation of ice sheets 52,53 . Compared to Mi-1, the extent and duration of cooling reflected in our records are weaker and shorter. Thus, we presume that from ca. 132. 49 Ma onwards, the polar summer continued to be cooler, and the ice sheets temporarily expanded with the appearance of minimum eccentricity and a sustained low amplitude in obliquity 52,53 . The evolutionary FFT spectrum and the power of eccentricity and obliquity can be correlated with the horizon of the permafrost wedges, which supports the accuracy of the revised palaeo-magnetic age framework (Fig. 7).

Cretaceous permafrost microbiome
A diverse complex of recognizable, and often exceptionally wellpreserved ancient filamentous, bacillar, and coccoid fossilized microorganisms was found in the Cretaceous permafrost wedges ( Fig. 8 and Supplementary Figs. [13][14][15][16]. Microbial fossils in the sample OR37b4 appeared to be more diverse in comparison to sample OR15b1 (Fig. 8). The fossils include remnants of prokaryotes (rod-shaped and coccoid bacteria, hyphae of actinomycetes, filamentous cyanobacteria), and aquatic eukaryotic unicellular microalgae (e.g., prasinophytes). The permafrost biome shows mineralized eukaryotic microorganisms gathered in monolayers where microbial cells were enveloped in sheaths and formed a film containing numerous oval cells about 1.4 μm long and 1-1.2 μm wide. Rounded cells with a diameter of about 1.4 μm and oval-flattened cells are also visible (Fig. 8a-c). Rod-shaped bacterial forms are 2.5-3.5 μm long and 0.4-0.5 μm wide, slightly curved with rounded ends and resemble Bacilli (Fig. 8d-f). All these sizes are in agreement with the most common dimensions of fossil bacteria, within the range of 0.2-2 μm, although maximum sizes of fossil bacteria can be >100 μm 54 .
Several lines of evidence indicate the mineralized stage of fossil bacteria and its fossilization. First, a lack of morphological variation and neither dehydration nor shrinkage of cells indicates a mineralized stage of fossil bacteria, as demonstrated by SEM images taken before and after the EDX analysis ( Supplementary Figs. 13 and 15). Second, the geochemical composition of analysed fossil bacteria reveals a C and O composition (carbon ca. 16-44%, oxygen ca. 25-43%) different from that of living organisms but very similar to lignite coal, peat and humic substances ( Supplementary Figs. 14 and 16). Third, geochemical EDX analysis demonstrates that this high C content occurs in fossils already Evidence of synsedimentary growth of bacterial colonies in the aeolian facies that host the sandstone wedges during deposition includes the widespread occurrence of mineral grains and cements covering microbial fossilized cells 54 (Fig. 8). Based on previous research 57, 58 we summarize that gram-positive bacteria, cyanobacteria, and unicellular eukaryotic algae are more often found as fossilized remains. Bacterial colonies, as those from the Cretaceous permafrost in China, indicate that the permafrost microbiome was organized in communities for efficient exploitation of energy and nutrients 54 in this extremely cold environment, where permafrost thaw might hamper trophic or cooperative interactions in the long-term among different microorganisms colonizing the permafrost 59 .
The fossils found in the Cretaceous permafrost wedges include prokaryotes (rod-shaped and coccoid bacteria, hyphae of actinomycetes, filamentous cyanobacteria), and aquatic eukaryotic unicellular microalgae (e.g., prasinophytes), resembling microbiomes from late Pliocene through Holocene permafrost 60,61 . Analyses of shotgun metagenomes from modern permafrost and the overlying active layer in Alaska 62 , in the High Arctic (Svalbard) 63 , and Canada 64 showed the presence of actinomycetes, filamentous cyanobacteria and eukaryotic algae. Siberian permafrost contains cyanobacteria and unicellular eukaryotic algae that can survive harsh permafrost conditions for millennia 61 . The preserved Cretaceous permafrost microbiome assemblage contains decomposers of organic matter such as actinomycetes, mycelial fungi, and bacteria, along with photosynthetic cyanobacteria and unicellular algae that controlled the net carbon dioxide emissions during Cretaceous permafrost thaw.

Discussion
Permafrost underlay a plateau desert in interior Asia during the Cretaceous supergreenhouse and was analogous to modern permafrost in a desert in the western Himalayas surrounding Qiongkuai Lebashi Lake. Identification of this past permafrost corroborates the palaeotemperature models for the Valanginian-Hauterivian stages 48 that predicted an annual mean surface air temperature (at 1.5 m) of ≤0°C for the Ordos Basin plateau desert basin and -8°C for the bounding desert mountain ranges (Supplementary Fig. 2b). The age of the permafrost sandstone wedges from the Luohe Fm is ca. 132.49-132.17 Ma (Hauterivian), based on the astronomical age constraint, and correlates with the occurrence of glendonites in Svalbard 65 , ca. 132. 36 Ma [boundary between B. Balearis and P. Ligatus biozones 66 ], ice-rafted debris (IRD) in Australia 6 , and glendonites and IRD in Alaska 67 , and slightly later than the Weissert event 68 (Fig. 7). The occurrence of wellknown Valanginian-age glendonites 13 , and IRD 9 , together with the identification of geochemical anomalies in western Tethys 69 points to extensive glacial ice in the Valanginian-Hauterivian, opening the door to the recognition of Valanginian permafrost in other polar and highaltitude paleolatitudes.
Major perturbations of the global carbon cycle during the Cretaceous have previously been attributed to increased levels of atmospheric CO 2 related to intensified global tectonic activity and/or widespread volcanic activity 70 . However, the effect of large igneous provinces (LIPs) seems to have a different correlation with the global variation of the palaeo-atmospheric pCO 2 71 and sea-surface temperature (SST) (°C) 72 . Moreover, the relative role of these controls has been re-evaluated and an alternate trigger for increased fertilization of the oceans should have existed 73 .
We hypothesize that global permafrost thaw during the Cretaceous released significant volumes of greenhouse gases to the atmosphere as well as dissolved organic carbon (DOC) and other nutrients into watersheds, and marine waters. Thaw of organic-rich permafrost increases carbon release and may affect aquatic systems through carbon and nutrient additions 5 . The contribution of permafrost thaw to the Cretaceous global C balance, including during oceanic anoxic events (OAE) will have to be determined in future research dealing with ocean-continental cryosphere coupling associated with events of cryosphere degradation in the aftermaths of supergreenhouse cold snaps.
Thaw of permafrost in plateau desert basins as that reported from the Cretaceous of China may constitute a neglected example of abrupt permafrost thaw 74 , postdating a global cold snap in the Hauterivian (Fig. 7) with plateau permafrost in China coeval with marine cryospheric indicators in Svalbard 65 , Alaska 67,75 , and Australia 6 . Global thaw of permafrost after the cold snap likely released carbon as greenhouse gases (CO 2 and CH 4 ) due to microbial decomposition of carbon previously frozen in permafrost, an example of positive feedback and likely to amplify climate warming 76 .
The development of plateau permafrost during the Hauterivian correlates with a global drop of SST (°C) 72 . Furthermore, its disappearance correlates with a global increase in atmospheric pCO 2 inferred from pedogenic carbonates 71 , with a global shift towards heavier values of δ 13 Cc (‰) 65 , coinciding also with a rapid rise of global SST (°C) 72 . The global synchroneity (interval 132.5-128 Ma) of the positive carbon isotope event, the SST variation, and the rise of atmospheric pCO 2 postdating the terrestrial record of plateau permafrost collectively points to a strong coupling of the ocean-atmosphere system. It also suggests that the disappearance of cryospheric systems associated with a global cold event may have affected the total exchangeable carbon reservoir.
Estimations made from mid-Pliocene (3.3-3.0 Ma) lacustrine systems from Tibet highlight that ca. 60% of alpine permafrost is vulnerable to thawing compared to ca. 20% of circumarctic permafrost 77 . These authors estimated that ca. 25% of permafrost carbon and the permafrost-climate feedback could arise from alpine areas 77 .
At present, permafrost stores~1600 BT of carbon 78 over 20% of Earth's terrestrial surface 62 . This store is nearly twice as large as the carbon stored today in the atmosphere 78 . We conclude that thaw of permafrost 76 after the Hautiverian cold snap promoted global warming of Cretaceous climates and fertilized the oceans. Antarctic evidence for glaciation of the South Pole during the Late Cretaceous 18 raises the likelihood that permafrost occurred at that time in Antarctica. Further investigation on recent permafrost analogues and of Cretaceous oceanic deposits may shed light on the exact contribution of permafrost thaw 79 to the radiative forcing of global Cretaceous events.

Fieldwork and sample collection
During our field investigations from October 8th to 15th, 2020, we conducted sedimentological analysis of aeolian sandstones in the Luohe Fm at the Yucha Grand Canyon, Ganquan County, Central Ordos Basin. We focused on the sandstone wedge structures within the aeolian sandstones of a well-exposed outcrop, OR15 (Supplementary Fig. 1c and d). The Yucha Grand Canyon is located on the north side of the Second Level Road of Ganquan County to Zhidan County, 40 km west of Ganquan County, and 72 km east of Zhidan County (Supplementary Fig. 1c and d). The entrance to the visitor center is in Zhangjiagou Village. From the visitor center, drive 3.7 km north to Yayaodi Village, and walk 366 m west to OR15 (Supplementary Fig. 1c and d).
As the outcrop OR15 with the wedges in the aeolian sandstone represents a steep cliff, we used a drone to investigate and photograph the upper portion of the outcrop. The drone model is the Mavic Air 2 (SZ DJI Technology Co., Ltd., Shenzhen, China).
For microscopy, samples from the wedges were collected with sterile tools and placed into sterile tightly sealed bags. In the laboratory, samples were ground up and prepared for microscopy inside a sterile box.

Natural gamma ray (GR) logging data
The intensity of GR levels in the rock relates to its content of uranium (U), thorium (Th), and potassium (K), reflecting the amount of clay and organic matter 80 . Potassium is concentrated in common minerals such as clays, feldspar, mica, and chloride salts. U and Th are commonly found in minerals such as clays, heavy minerals, feldspars, and phosphate, whereas U is usually concentrated in organic matter 80 . The distribution of these radioactive elements, determined from GR data with the advantage of continuous and high resolution, has been used as a palaeoclimatic proxy for many cyclostratigraphic studies 81,82 . GR logs of Well Wuqi were selected for cyclostratigraphic analysis of the early Cretaceous in Ordos Basin (Supplementary Data 1 and 2).

Time series analysis and modelling
Time series analysis of the GR data was performed with Acycle 2.0 software 83 . The long-term linear trend in GR data was removed by the detrending function. The 2π multitaper (MTM) method of spectral analysis with a red-noise null model was performed to detect significant frequency peaks. Sliding-window analysis with the evolutionary fast Fourier transform (FFT) method was used to examine changes in dominant frequency patterns. The Gaussian bandpass filter was used to filter 405-kyr signals. In addition, the correlation coefficient (COCO) analysis was applied to evaluate the optimal sedimentation rate for the studied sequence 83 . The La2004 orbital solution has been identified as a precise astronomical target for the period 134-129. 4 Ma 84 .

Scanning electron microscope (SEM) analysis
The SEM instrument model was SIGMA300 (Carl Zeiss AG, Germany) and the whole experiment was carried out at the Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Chengdu, China. A total of 9 samples for SEM analysis was collected. The sandstone wedge structures cropping out in the field were gently tapped into small blocks to obtain the natural section and then processed to a size of 1.5 cm*1.5 cm. The samples were fixed to the sample stage (1.3 cm*1.3 cm) by means of conductive adhesive and coated using the gold plating instrument with three sprays of 10 min each. Three samples were placed in the SEM sample compartment at a time. Sterile gloves and masks were worn throughout the process and the samples were cleaned repeatedly with compressed air to ensure they were not contaminated. The SEM was operated at 10 kV, HDBSD mode. The sample number was specified on the operating screen and the microbial phenomena were photographed at different magnifications after the optimum photographic definition was obtained by adjusting the axis distance, contrast, brightness, and focus. The energy dispersive X-ray spectroscopy energy spectrum was scanned as a point and surface scan, with the Au elements, removed and normalized to give the results. Photomicrographs were taken before and after the energy spectrum scan to observe any changes in microbial morphology and make sure the microbes observed were fossils and that no morphological changes occurred after the electron beam was fired.

Data availability
The Gamma Ray log data of Wells Wuqi and Lingtai of the Cretaceous Luohe Fm. used in this study are available in the University of Sussex data Repository under accession code https://doi.org/10.25377/ sussex.21610635.